It often depends on which PMS system the hotel is using. Not every hotel uses the same PMS system, some do not have the capability to email receipts instantaneously at check out, some do, some do not have the capability at all yet, and those guests that request the email have to wait for the processing system in Rewards to do its thing.
Most of those that do have the capability are stopping delivering receipts, those that don't continue the practice. On top of that, it is down to lack of consistency between the huge number of franchisee's, some owners cut the costs involved out some don't.
Marriott is rolling out new PMS systems to all properties that currently have the outdated systems, but this is an enormous project with over 4,000 properties to co-ordinate, its not just a case of turning up with a new computer.